1. Field of the Invention
The present invention relates to a liquid crystal display device used for displaying images by using alignment characteristics of liquid crystal and a liquid crystal display for displaying images by utilizing the liquid crystal display device, particularly to a liquid crystal display device and a liquid crystal display which utilize vertically aligned liquid crystal.
2. Description of the Related Art
In recent years, small projection displays with high definition and high luminance have been increasingly demanded. Accordingly, as a display device for the projection display, a reflective device has been in practical use. By using the reflective device, a small projection display with high definition and high light use efficiency can be realized.
As the reflective device, an active reflective liquid crystal display device having a structure in which liquid crystal sandwiched by alignment films is enclosed between a semiconductor driver substrate and a transparent electrode substrate is known. The semiconductor driver substrate is structured in such a manner that a drive device for driving a circuit, a reflective electrode for reflecting light (so-called pixel electrode) and the like are provided on a semiconductor substrate. The transparent electrode substrate is structured in such a manner that a transparent electrode and the like are provided on a transparent substrate. The alignment film is intended to align liquid crystal molecules to a given alignment state.
In the reflective liquid crystal display device, when a voltage is applied between the pixel electrode and the transparent electrode, the alignment state of liquid crystal molecules is changed according to an electric potential difference between the electrodes, that is, the optical properties of the liquid crystal are changed. Therefore, light is modulated by utilizing such change in the optical properties of the liquid crystal. Thereby, gradation is performed based on the modulated light to display images.
Of the reflective liquid crystal display devices, in particular, a reflective liquid crystal display device utilizing liquid crystal having vertical alignment (so-called vertically aligned liquid crystal) has high contrast and high response speed. Therefore, such a device has attracted attentions as a device capable of improving display performance. The “vertically aligned liquid crystal” is a liquid crystal having negative dielectric anisotropy, that is, having characteristics that a difference δ∈ (=∈(∥)−∈(⊥)) between a dielectric constant ∈(∥) in a direction parallel to a long axis of liquid crystal molecules and a dielectric constant ∈(⊥) in a direction perpendicular to the long axis of liquid crystal molecules is negative.
In the case that such vertically aligned liquid crystal is utilized, liquid crystal molecules are vertically aligned with respect to the substrate surface of the semiconductor driver substrate when an applied voltage is zero, and therefore a display state called normally black mode is obtained. Meanwhile, when a voltage is applied, liquid crystal molecules are tilted to the substrate surface, and therefore light transmittance is changed. In this case, in particular, when the tilt directions of the liquid crystal molecules are not in the same direction, the contrast becomes unevenness. To prevent such unevenness, it is necessary to align the liquid crystal molecules at a slight angle (pre-tilt angle) in advance in a certain direction.
As a method of aligning the vertically aligned liquid crystal so that a desired alignment state is obtained, there are two methods as follows. First, there is a method in which an organic alignment film typified by polyimide is used, more specifically the organic alignment film is rubbed and the alignment state is thereby controlled. Secondary, there is a method in which an inorganic alignment film typified by silicon oxide is used, more specifically the inorganic alignment film is obliquely evaporated and the alignment state is thereby controlled. Of the foregoing methods, the inorganic alignment film has attracted attentions as an alignment film capable of realizing high luminance of the foregoing projection display. In these days, there is a tendency that light source power is increased to realize high luminance of the projection display, that is, there is a tendency that alignment films are exposed to high power light. Considering such a tendency, to secure a display performance of the projection display for a long time, the inorganic alignment film with high light resistance is more preferably used than the organic alignment film with low light resistance. In the case of using the inorganic alignment film, when silicon oxide is obliquely evaporated, the pre-tilt angle can be controlled by changing the incident angle of the evaporation particles.
For a structure of the alignment film mounted in the reflective liquid crystal display device and a method of forming it, a plurality of techniques have been already known. Specifically, a technique in which a first obliquely evaporated alignment film is formed by obliquely evaporating silicon oxide from a first oblique evaporation direction so that a side surface of one of two pixel electrodes adjacent to each other is covered, and then a second obliquely evaporated alignment film is formed by obliquely evaporating silicon oxide from a second oblique evaporation direction so that a side surface of the other pixel electrode which is opposed to the side surface of the former pixel electrode is covered has been proposed (for example, refer to Japanese Unexamined Patent Application Publication No. 2005-084586). According to the technique, the peripheral structures of the side faces of the two pixel electrodes become symmetric to each other. Therefore, when the reflective liquid crystal display device is operated for a long time, it becomes difficult to generate a burn-in phenomenon resulting from asymmetry of the peripheral structures.